Production of electrolytic zinc



the process.

Patented Mar. 23, 1954 UNITED STATES PATENT OFFICE 2,673,180 PRODUCTION OF ELECTROLYTIC ZINC Walter Arthur Sweeney,

conda, Mont., assi Neill New York, N. Y., a

No Drawing. Application November 3, 1950,

Serial N o.

Claims. (Cl. 204-119) This invention relates to the electrolytic production of metallic zinc, and is particularly directed to an improved procedure for the treatment of the zinc-bearing electrolyte to produce a metallic zinc of very low lead content.

The process most commonly used for the production of electrolytic zinc involves passing an electric current between a lead anode and an aluminum cathode through an acidic zinc sulfate electrolyte. The metallic zinc thereby deposited on the cathode is of very high purity, often upwards of 99.99% zinc. The only impurity of consequence generally present is a very small amount of lead, which evidently finds its way from the lead anodes into the electrolyte and thence into the zinc deposit. Although the quantity of lead present in the best grades of electrolytic zinc is measured in thousandths of 1%, it is nonetheless considered highly objectionable for some uses to which the zinc is put, particularly in the making of zinc-base die-casting alloys. There is a large demand for zinc metal known commercially as special high grade, the specifications for which require that the lead content be no greater than 0.005% when the metal is to be used for making zinc-base die-casting alloys, and some purchasers insist on an even lower lead content.

One procedure heretofore developed for inhibiting the inclusion of lead in electrolytic zinc comprises introducing a small amount of a strontium compound into the zinc sulfate electrolyte. Introduction of the strontium in the form of an aqueous slurry of strontium carbonate has been considered the most effective and economical manner in which to make use of this procedure in commercial operations. However, the amount of strontium carbonate required to insure that the zinc will be safely Within special high grade make its cost specifications is large enough to a substantial element in the cost of Moreover, the strontium carbonate slurry is rather difficult to handle. Its content of solids tends to settle out and plug distribution pipes and delivery orifices. In some commercial operations, the only satisfactory way for introducing strontium carbonate slurry into the electrolyte has been to ladle it, a cupful at periodic intervals, into each electrolytic cell. This procedure is of course costly, and suffers from the drawback that uniform distribution of the strontium compound throughout the electrolyte is not achieved. In spite of these disadvantages, the strontium carbonate slurry has been found both more elfective and more economical to use than responding to about three pounds solutions of soluble strontium compounds such as the chloride.

We have discovered that a solution of strontium sulfate in concentrated sulfuric acid is a much more effective vehicle for the incorporation of strontium into the zinc electrolyte than is the strontium carbonate slurry heretofore preferably used. We have found that by delivering the strontium to the electrolyte in the form of strontium sulfate solution in concentrated sulfuric acid, the quantity of strontium compound required for producing electrolytic zinc of given low lead content is only about one third of the amount required when the strontium is delivered to the electrolyte in the form of a strontium carbonate slurry. Further, such solution is: much more convenient than the carbonate slurry to handle and deliver into the electrolyte.

Based on these discoveries, our invention provides the improvement, in the above-mentioned process for producing electrolytic zinc, of introducing into the acidic zinc sulfate electrolyte a solution composed essentially of strontium sulfate dissolved in concentrated sulfuric acid. The concentration of strontium sulfate in such acid solution is not critical, but for reasons of economy the solution is made and used in as concentrated form as is convenient to prepare. Likewise the amount of strontium sulfate introduced into the electrolyte is not critical, but for an optimum combination of effectiveness in inhibiting lead contamination of the zinc with economy in commercial use, we find it desirable to introduce the solution into the electrolyte in an amount corof strontium sulfate per ton of zinc deposited. The strontium sulfate solution is not subject to the distribution difficulties inherent in the use of a strontium car bonate slurry, and in accordance with our invention such solution may be delivered continuously into the electrolyte as it flows to the cells in which the electrolysis is carried out.

The manner in which the added strontium sulfate solution functions to inhibit the inclusion of lead in the cathode zinc deposit is not known,

though there are indications that it probably causes precipitation of lead from the electrolyte as a double sulfate salt of lead and strontium. Regardless of the form in which the strontium is delivered to the zinc electrolyte, whether as strontium carbonate or strontium chloride or other strontium compound, it doubtless is promptly converted to the very slightly soluble strontium sulfate as soonas it is incorporated in a out 0.003%,

the zinc sulfate electrolyte, which is quite fate to the electrolyte has no significant effect on the lead content of the zinc deposit, and 'by the further fact, on which this invention is based, that a solution of strontium sulfate in concentrated sulfuric acid is approximatlyfthree times as effective for controlling thesl'ead content of the deposit as is the strontium carbonate slurry heretofore used even when the modeofintroducing the two strontium compositions is the "same.

The most satisfactory way in which to prepare the strontium sulfate solution for use in accordance with the invention is by dissolving the mineral celestite (anaturally occurring form of strontium sulfate) in concentrated sulfuric acid. Althoug h chemically precipitated strontium sulfate has long been ,lgnown to be soluble concentrated sulfuric acid, strontium sulfate-in the form of celestite has heretofore been considered substantially insoluble in acids. Ngnethelss, we have 'found that the strontium snfatecontent of natural celestite dissolves in 98% sulfuric acid to the extent of about by weight of the acid used, and dissolves in 93% sulfuric acid tothe extent of about 8% by weight pf the acid used. In preparing the solution,the celestite in finely divided form is mixed with sufficient concentrated sulfuric acid to dissolve substantially all ofthe strontium sulfate, but no more acid isused'than is required to make about as concentrated aj'solution as is convenient to prep 're. The mixture"then is stirred for a sufiicient period of time to effect substantially complete dissolution of the strontium "sulfate. n

desiredfthe acid may be heated to facilitate dissolution. Any remaining insoluble residue is advantageously separated by filtration or decantation, and the resulting larified somuon is then ready for delivery into the zinc electroit o The preparation and use of a concentrated solution of strontium "not necessary for effectiveness in controlling the laii centers of the zincfbut is desirable to minize "the amount er acidadded to the electrciyte. I y In a conventional electrolytic 'zinc plant a stream of the acidic zinc sulfate electrolyte flows continuously to and through a series of electro'lytic cells in which it passes between anfarray of leadanod'es and aluminu'm'c'athodes. The

strdritiumkulfate solution prepared as described above may be introduced substantially continuously into the zin'c electrolyte as itflows to the electrolytic cells; or alternatively it may be introduced into the solution in each "cell.

In one commercial electrolytic zinc operation, in which a slurry of strontium carbonate in water was incorporated in the electrolyte to inhibit the inclusion of lead in the cathode zinc deposit, approximately ten pounds of strontium carbonate perton of zinc produced was required to keep the lead content of the zinc deposit at H Upon substituting a solution of strontium sulfate in concentrated sulfuric acid, but without any other change in the process, only about three pounds of strontium sulfate per ton of zinc deposited was required to obtain the same result. It is thus 'apsaieutuintue sulfate "in sulfuric acid is a an acidic zinc sulfate electrolyte of strontium sulfate is somewhat more than three times as effective as the strontium carbonate slurry, on the basis of the weight of strontium required, for controlling the lead content of electrolytic zinc. In addition, 'tlie"'strontium sulfate solution is less costly to prepare than the carbonate slurry, and *issubstantially easier and less expensive to handle, and to deliver into the electrolyte.

We claim:

*1. In the production of electrolytic zinc involving the passage of an electric current through from a lead anode toatathoue on which metallic zinc is deposited, and wherein lead collects as an impurity in the electrolyte and tends to be deposited as an-impurity in the metallic zinc, the improvement which comprises introducing into the leadbearing electrolyte a solution composed essentially of strontium sulfate dissolved in concentrated "sulfuric acid, whereby the incorporation of, lead in the'ca'thod'e zinc deposit is inhibited.

2. ma process "forproducing metallic zinc electrolytically, in 'whicha'n electric current is passed through an acidic "zinc sulfate electrolyte between a lead anode and a cathode on which metallic zinc is deposited, and wherein lead c01- lects as an impurity in the electrolyte-and tends to be deposited as "an impurity in the metallic zinc, the improvement which comprises introducing into the lead-"bearing electrolyte a solution of strontium sulfatein concentrated sulfuric acid, such solution being introduced into the electrolyte in an amount corresponding to about three pounds of strontium sulfate per ton of zinc deposited, whereby the incorporation of lead in the cathode zincdeposit is-inhibited.

3. In a "proces's for producing metallic *zinc electrolytically, in which a'stream of acidic zinc sulfate electrolyte is flowed continuously between an array of lead anodes and cathodes on which metallic zinc is deposited electrolytically, and in which lead collects as an impurity in the electrolyte and 'te'ndsjto be deposited as an impurity in the metallic zinc the improvement which comprises substantially continuously introducing into the flowing stream of lead-bearing electrolyte a solution composed essentially of strontium sulfate dissolved in concentrated sulfuric acid, whereby the incorporation of lead in the cathode zinc deposit is inhibited.

4. In a process for producing metallic 'zinc electrolytically,in "which'a stream of acidic zinc su'lfate electrolyte is flowed continuously between anarray of lead anodes and cathodes on which metallic'zinc is deposited electrolytically, and in which lead collects "as an impurity'in theelectrolyte and tends to be deposited as an impurity in the metallic zinc-the improvement wuich'comprises substantially continuously introducing into the flowing stream of lead-bearing electrolyte a solution composed essentially of strontium sul: fate dissolved in concentrated sulfuric acid, such solution being delivered into the stream of elec-; trolyte at a ratecorresponding to about three pounds of strontium'sulfa-te-per ton of zinc =deposited, whereby the incorporation of lead in the cathode zinc deposit is inhibited.

5. In a process for producingmetalli'c zinc of very low leadconten-t by electrolysis "'of arrac'id-ic zinc sulfate electrolyte between'lea'danodes and cathodes'on which the-zinc isdeposited, inwhich lead collectsfas an impurity in the electrolyte and tends to be deposited as an "the sulfuric acid solution cathode zinc deposit, and in which a strontium compound is incorporated in said electrolyte to inhibit incorporation of lead in the cathode zinc deposit, the improvement which comprises introducing the strontium into the lead-bearing electrolyte by adding thereto a solution of strontium sulfate dissolved in concentrated sulfuric acid.

WALTER ARTHUR EMANUEL.

KENNETH ONEILL SWEENEY.

JOHN FRANCIS MAHONEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,539,681 Yeck et a1 Jan. 30, 1951 OTHER REFERENCES 

1. IN THE PRODUCTION OF ELECTROLYTIC ZINC INVOLVING THE PASSAGE OF AN ELECTRIC CURRENT THROUGH AN ACIDIC ZINC SULFATE ELECTROLYTE FROM A LEAD ANODE TO A CATHODE ON WHICH METALLIC ZINC IS DEPOSITED, AND WHEREIN LEAD COLLECTS AS AN IMPURITY IN THE ELECTROLYTE AND TENDS TO BE DEPOSITED AS AN IMPURITY IN THE METALLIC ZINC, THE IMPROVEMENT WHICH COMRISES INTRODUCING INTO THE LEADBEARING ELECTROLYTE A SOLUTION COMPOSED ESSENTIALLY OF STRONTIUM SULFATE DISSOVLVED IN CONCENTRATED SULFURIC ACID, WHEREBY THE INCORPORATION OF LEAD IN THE CATHODE ZINC DEPOSIT IS INHIBITED. 